The first successful hematopoietic stem cell transplantations (HCT) were done in 1968 in three children with congenital immune deficiency diseases. In each instance, stem cells were collected from the bone marrow of sibling donors who were genotypically identical or closely matched to the recipient for human leukocyte antigens (HLA). Since then, thousands of HCTs have been performed to treat malignant and non-malignant diseases. Current estimates of annual numbers of HCTs are about 45,000 worldwide, with 15-20,000 annually in the United State. Reasons for increasing use include proven and potential efficacy in many diseases, better understanding of the appropriate timing of transplantation and patient selection, greater availability of donors and better techniques for determining HLA match, greater ease of stem cell collection, and improved supportive care resulting in less transplant-related mortality. Despite these improvements, application of HCT is still limited by relatively high risks of morbidity and mortality and, in the case of allogeneic HCT, availability of suitable donors. HCT outcomes are influenced by many patient- and disease-related factors such as age, disease stage and prior treatment, as well as transplant-related factors such as stem cell source and transplant regimen. Ideally, the effectiveness of most transplant strategies would be evaluated in large randomized clinical trials. However, various factors hamper the ability to conduct randomized trials in this setting. Many diseases treated with HCT are uncommon. Additionally, for allogeneic transplants, not all patients have a suitable donor identified. Consequently, even when one considers the most common indications for HCT, numbers of eligible patients in individual centers are small. The ability to test new transplant strategies in a definitive way requires the existence of a network committed to development and implementation of large, multicenter studies. Under RFA HL-01-004, the National Heart, Lung, and Blood Institute (NHLBI) and the National Cancer Institute (NCI) jointly established a Blood and Marrow Transplant Clinical Trials Network (BMT CTN). The goal of the Network is to efficiently assess novel HCT methods and management strategies derived from single center studies in a multi-center setting. The focus of this group, in contrast to cancer clinical trials groups, is on improving the transplant procedure per se rather than cancer treatment in general. (End of abstract.)